Recently new types of permanent magnets have become available with significantly increased energy products. These new magnets comprise alloys of a rare earth (usually neodymium or praseodymium), iron, and a promoter of metastable phases (such as boron or gallium). For example, see application Ser. No. 470,968 filed Mar. 1, 1983, "Permanent Magnets and Method of Making Same", by Hazelton and Hadjipanayis assigned to the assignee hereof. Prior alnico (aluminum, nickel, cobalt) magnets usually have an energy product in the range of 5-7 MGOe, samarium-cobalt SmCo.sub.5 magnets have an energy product of about 17 MGOe and the more expensive samarium-cobalt Sm.sub.2 Co.sub.17 magnets have an energy product of about 27 MGOe. By comparison, Ne Fe B (neodymium, iron, boron) magnets are now available, for example, from Sumitomo Special Metals of Japan, with energy products in excess of 35 MGOe.
A great many motor designs have been created in the past, many taking advantage of improved permanent magnet characteristics. Slotted motor structures have been the most common in which copper windings are placed in laminated iron slots. The slotted designs provide a motor with a relatively small air gap in the magnetic circuit to achieve a desired high permeance. Magnets with increased energy products (e.g. samarium-cobalt) have proportionately reduced the magnet mass and resulted in effective inside-out brushless designs with rotating magnets and windings on the stator.
Non-slotted designs are also known where the windings are located in the air gap. Such slotless designs have proven effective primarily in large turbogenerators where expense is no object if it achieves improved operating efficiency. These turbogenerators employ sophisticated cooling systems and super-conducting materials to achieve high flux densities across a large air gap which accommodates the windings. Slotless designs in small motors have also been proposed but these have usually been either special purpose (e.g. high speed toroidally wound motors) or low performance motors not suitable for servo applications.
An object of the present invention is to provide a motor design which can make effective use of high energy product permanent magnet materials.
A more specific object is to provide a motor design for effectively using permanent magnets like the available NeFeB magnets having an energy product above 26 MGOe and preferably above 30 MGOe.
Still another object is to provide a method of making a high performance motor with windings located in the motor air gap.